1. Field of the Invention
This invention relates to a method and apparatus for detecting the lubricant type and degree of water contamination present in a lubricating oil. Additionally, the invention provides a method for classifying unknown oils as to additive content.
2. Description of the Prior Art
Lubricating oils serve multiple functions in mechanical systems. They reduce friction in each of the three lubrication regimes depending on application type: hydrodynamic, elastohydrodynamic, and boundary.
For applications with conformal bearings, such as Babbitt bearings, the lubricant transfers distributed loads, in the form of radial pressure, from shaft to sleeve. This is hydrodynamic lubrication and is similar to an automobile hydroplaning on wet pavement.
For highly loaded rolling element bearings such as gears and anti-friction bearings, the viscosity of a protective lubricant must increase exponentially with pressure which results from forcing the lubricant into the small clearance between the rolling element and the raceway. The lubricant viscosity increases until the oil has the bulk modulus-of-elasticity of a solid, able to transmit highly concentrated loads. This form of lubrication, elastohydrodynamic, practically eliminates metal to metal contact with extremely low friction energy losses.
For traction type applications such as slides and wheels, lubricant serves to reduce wear due to sliding contact and abrasion. In this case, the physical loads are transmitted through metal to metal contact. The traction or friction in this type system can be very high, with coefficients of sliding friction as high as 1/3 of the normal force that presses the two components together. The friction, wear and traction may be greatly reduced by lubricating the surfaces so that only a limited amount of shear can be transmitted between components. This can be done with either solid lubricants such as Teflon, molybdenum disulfide or graphite or with liquid lubricants formulated with extreme pressure and anti-wear additives.
In addition to these requirements for transferring loads, reducing friction, and minimizing wear, lubricants fulfill several other functions critical to machine life. These include sealing surfaces from corrosion and other forms of chemical attack, transmitting hydraulic power, cleaning away contaminants and wear debris, cooling hot surfaces and electrically isolating and insulating dissimilar metals from galvanic interaction. In the special case of electric transformer applications, oil provides cooling and electrical isolation (i.e., a low dielectric medium).
Moisture contamination of lubricating oil is a frequent and serious problem for many industrial plant applications. Moisture causes problems by rendering the oil additives ineffective, by corroding metal surfaces, and by incapicating elastohydrodynamic lubrication. Sources of oil contamination by water include the environment (humidity, rain and condensate for example), process materials (fluids and steam) and facility cleaning operations.
Since water is substantially immiscible with petroleum oil, chemically free water, in fluid presence with oil, stratifies in the low points of a reservoir or circulation system. If the system is turbulent, such free water may entrain the oil as droplets. In either case, the water displaces the oil to prevent the lubrication function of the oil and jeopardize the mechanical system. Moisture also chemically reacts with many oil additives otherwise intended to provide antiwear, antioxidation, extreme pressure (EP), anti-foaming and detergency functions. Furthermore, moisture consumes all or part of the dispersant and emulsifier additives intended to prevent contaminants from coalescing or agglomerating.
It is well known that moisture promotes corrosion of metal surfaces. The polar nature of the water molecule supports ionic mobility and encourages chemical attack to copper, lead and other reactive metal surfaces, particularly in the presence of air. Moisture also supports electrochemical interaction between galvanically dissimilar oil wetted parts.
As mentioned previously, elastohydrodynamic lubrication depends on the unique pressure-viscosity property of mineral oils and synthetic lubricants. Free water droplets are unable to maintain physical separation between rolling elements or between gear teeth under these extremely high pressures. The resulting impacts cause surface damage and shorten component life.
There are three forms or states in which water is found when combined with oil. The water can be dissolved in the oil in the sense that it is hydrated, dissolved or reacted with additives that are mixed with the oil. In another state, oil can be dispersed or emulsified with the oil. Finally, it can be independently stratified or mixed as free water droplets which are separate from the oil. Each of these states damages the oil and the mechanical system in a different way.
Water is a polar molecule whereas hydrocarbons are non-polar. Since `like dissolves like` the solubility of water in hydrocarbons is small. As the size of the hydrocarbon increases (that is the number of carbon atoms in the chain), the water solubility decreases. Aromatic hydrocarbons will have a higher water solubility than paraffinic hydrocarbons. Therefore, hydrocarbon oils will have water solubilities from less than 1 to about 100 ppm.
Oils used in industry may contain additives or be entirely synthetic (e.g. PAG, esters etc.). Generally the water solubility increases with increasing oxygen content based on elemental analysis. An oil dielectric constant also increases with increasing oxygen content. A more sophisticated analysis would include contributions from sulfur, phosphorous and metals. These minor constituents would increase the dielectric constant and increase the solubility of water. A rough rule of thumb is that as the dielectric constant increases, the water solubility should also increase.
Moisture dissolved in the oil consumes performance enhancing additives and promotes corrosion. Reaction of water with oxidation inhibitors produces acids and precipitates. These water reaction products increase wear and interferences. At high operating temperatures (above 60.degree. C.), water reacts with and destroys zinc type antiwear additives. For example, zinc dithiophosphate (ZDTP) is a popular boundary lubricant added to hydraulic fluid to reduce wear in high pressure pumps, gears and bearings. When this type additive is depleted by reacting with water, abrasive wear accelerates rapidly. The depletion shows up as premature component failure resulting from metal fatigue and other wear mechanisms. Other reactions produce extreme pH compounds which secondarily react with metal components of the machine the lubricant is intended to protect.
Emulsions are "significantly stable" complexes of two immiscible liquids. The term "significantly stable" relates to intended use and may range from a few minutes to years. Two types of emulsions are recognized: macroemulsions and microemulsions. Macroemulsions, which are more common, range from 0.2 to 50 micrometers and are easily visible under a microscope. Microemulsions range from 0.01 to 0.2 micrometers and are not visible Under a microscope. The size of the dispersed particles in an emulsion determines its appearance to the naked eye. The diameter of the dispersed particles in an emulsion determines its appearance to the naked eye. If the diameter of the dispersed particles is 1 micrometer, it appears milky white; 1-0.1 micrometers, blue white; 0.1-0.5 gray and semitransparent; &lt;0.05 micrometers it is transparent (Surfactants and Interfacial Phenomena, M J Rosen, John Wiley & Sons, 1979, p224). The smaller the range of sizes of droplets in the emulsion, the more stable it is. Thus, macroemulsions are opaque and microemulsions are transparent to semitransparent to visible light.
Two immiscible pure liquids can not form a `stable` emulsion. In order to form a stable emulsion, a third component or an emulsifying agent must be added. This agent may be a surfactant, although surface active agents may include finely divided solids. Frequently the most effective emulsifying agents are mixtures of two or more substances which act synergistically. The most common combination of emulsifying agents are a water soluble surfactant mixed with an oil soluble surfactant.
Water emulsified with oil consumes nearly all remaining additives, increases corrosion and changes fluid viscosity (normally increasing it). Systems designed to operate with water present must either emulsify or demulsify the water to extend life of the mechanical parts. Automotive applications must emulsify the water until it can be driven off by heat. Steam turbines typically demulsify water, dropping it out in the oil compartment. In either of these cases, water left in the system can lead to corrosion and even microbial growth. Under certain conditions, bacteria can live and reproduce very rapidly when sufficient water is present in oil. These living organisms can ruin the oil system by clogging filters, changing water emulsion characteristics, increasing corrosion rates and producing acidic waste products.
Free water forms from the coalescing of emulsified water. It is the thermodynamically stable state with oil that, is supersaturated with water. In oils with no additives (e.g. turbine oils), free water forms rapidly. In oils with additives that form stable emulsions, free water forms after the emulsifying agent is saturated by water. Free water droplets are desirable in some lubricating systems (those designed to demulsify water) and very detrimental in others.
In mineral oils and polyalphaolephin (PAO) synthetic hydrocarbon oils, additives are required,for any significant amount of water either to be dissolved in oil or to be emulsified with oil. Pure mineral oils are saturated with as little as 1 ppm water, and turbine oils saturate with as little as 100 ppm water. Above saturation level, water coalesces into free water droplets eventually settling to the bottom of the oil compartment. This is a very desirable characteristic for most steam turbines and paper machines. In these applications, water contamination is common. So to avoid the consequences of moisture retention in these machines, oils are designed to demulsify, or drop water out in the oil compartment before returning to the machine. Periodically, water is bled off from the bottom of the oil compartment (e.g., sump). Demulsibility of oil is good if water Separates quickly at operating temperatures. It is bad if it separates slowly. Oils designed to demulsify oil are normally pure mineral oil or PAOs with very little additive.
Engine oils and most other industrial lubricants will tend to emulsify rather than demulsify water. In these lubricants the additives serve to disperse water and prevent it from coalescing into free water droplets.
Additives modify the solubility and emulsion character of mineral oils. Transformer oils are saturated with &lt;1 to 3 ppm water. Oil-based hydraulic fluids are typically saturated with 100 ppm (0.01%) to 1000 ppm (0.1%) water Industrial lubricants are typically saturated with 600 ppm (0.06%) to 5000 ppm (0.5%) water. Automotive lubricants are typically saturated with 1% to 5% water. Stern tube oils (those used to lubricate the aft bearing on a ship propulsion shaft), have increasing amounts of additive, particularly increasing amounts of total dispersant, detergent, anti-oxidant, anti-wear and extreme pressure compounds.
Many methods and devices have been developed to detect the contamination or breakdown of oil. One such device, shown in U.S. Pat. No. 4,646,070 issued to Yasuhara, discloses a device for detecting deterioration in lubricating oil which comprises a pair of capacitor electrodes positioned in the lubricating oil. The device uses the oil as a dielectric between the electrodes to develop a voltage frequency signal across the sensor capacitor thus determining the dielectric number of constant and deterioration of the oil. A major drawback of this device and others is that they do not inform the tester of the Specific type or magnitude of deterioration in the system.
Other methods of oil analysis comprise the preparation of microscope slides whereby particles are counted, measured and visually evaluated for subjective clues to wear, breakage and failure. Although effective, this type of analysis is both slow and expensive.
Lubrication oil is frequently blended with non-petroleum additives such as detergents to modify the oil properties. In the presence of water, these additives may combine with water, either reactively in solution or passively as emulsions. In either case, the additives are prevented from their objectives of improving the lubrication properties.
When water is present in a lubrication oil system, it is useful to know the quantity and form of such water. Moisture in solution may be measured using a wet chemistry filtration method. Although effective, this type of analysis is both slow and expensive.
It is, therefore, an objective of the present invention to provide a method and apparatus for quickly identifying the presence of moisture in a lubrication oil system.
Another object of the invention is to provide a method and apparatus for identifying the solution dispersed or free form that moisture in a lubrication system may be found.
Another object of the invention is to provide a method and apparatus for identifying the percentage of moisture in a lubrication system in each of three states of combination.
Another object of the invention is to classify the type of lubricant for the purpose of automatic oil analysis based on dielectric number or dielectric change due to temperature.